This is an open label, non-blinded, proof of concept efficacy and safety study of eculizumab in patients with biopsy proven DDD or C3 nephropathy. The trial will consist of adult patients with these diseases who have > 1 gram of proteinuria or a decreased glomerular filtration rate (GFR), both predictors of a poor long-term outcome in many glomerular diseases. They will be treated with eculizumab for one year. The goals will be to determine whether treatment leads to an improvement in kidney function, defined by remissions of proteinuria and improvements in estimated GFR (measured by serum creatinine), and to improvement in histologic parameters, including percentage of non-affected glomeruli, interstitial fibrosis, intensity of C3 staining of immunofluorescence, and amount of electron dense deposits by electron microscopy.